In order to increase productivity in a manufacturing operation that requires heating and cooling of an article as part of the manufacturing process, accelerated heating or cooling can be effective, as long as the accelerated heating or cooling does not affect the properties of the article in its final condition.
For example, vacuum heat treating is becoming increasingly more important to the metal treater because of environmental concerns. While in the past open heating methods such as salt bath heaters have been used, environmental concerns have pushed the heat treater to heating and vacuum where the parts to be heat treated can be brought to temperature under vacuum conditions, thus preventing surface oxidation and/or surface pickup of unwanted contaminants from the atmosphere, while preventing release of unwanted components driven off in the heating process into the environment.
Conventional and state-of-the-art vacuum furnaces are available from a number of companies including Abar Ipsen Industries headquartered in Feasterville, Pennsylvania, which provide a device for the heat treater to bring parts to elevated temperature above the critical temperature at which isothermal transformation takes place. In the case of ferrous alloys, heating the metal above the transition temperature followed by rapid cooling can produce a part with extreme hardness. One method of quenching is by backfilling the vacuum furnace with an inert gas in order to cool the part back to a temperature where the surface of the part be unreactive to ambient atmosphere, once the soak temperature has been reached and the part held for sufficient time. However, in the past the only way to improve cooling rates was to make expensive modifications to the furnace in order to increase the quench pressure and blower speeds to maximize the cooling rate. Even with such modifications, the heat transfer properties of the inert quench gas remain a major limiting factor.
The U.S. Pat. No. 4,643,401 discloses a complex method using various gases cooled to cryogenic temperatues to achieve a quench after vacuum heating.
It is well known that many of the new alloys used for aerospace applications and alloys with larger cross-sections present vacuum heat treaters with problems in achieving required cooling rates utilizing furnaces of existing design. Currently two methods of increasing the cooling rate of existing equipment involve either expensive retrofits to the furnace as set forth above and/or increasing the heat transfer properties of the cooling medium. Among the many options available in retrofitting the furnace, the most popular are to increase the blower capacity of the vacuum system or to purchase an advanced plenum for more uniform and turbulent impingement of the cooling gas on the part. It is estimated that retrofit options can have a price tag in excess of $50,000 and a complete vacuum furnace replacement can cost in excess of $400,000.
Most vacuum heat treaters currently use nitrogen or argon for a quenching gas. If they required faster cooling rates, until the present invention, the only non-combustible alternative was to change to helium. While helium has the best relative cooling ability of the three gases (argon, nitrogen, helium) it has a price tag that is considerably higher than argon or nitrogen.
In the past, vacuum treaters have considered retrofitting or replacing their existing equipment, however, the cost is often prohibitive. Many titanium and high chromium alloys require very rapid cooling in order to meet material specifications. These metals require the use of argon in place of nitrogen since they are subject to nitrogen pick-up on the surface of the part during heating. The disadvantage is that argon has the poorest heat transfer characteristics. For large cross-section parts (e.g., greater than 3 inches) rapid inert cooling in vacuum furnaces has been extremely difficult. Also, current vacuum furnace technology prohibits many heat treaters from processing high speed steels in vacuum furnaces. The heat treaters are forced to put lighter loads in the furnace which significantly reduces production capabilities.